A conventional plate heat exchanger is made up of a frame which supports two flexurally rigid clamping plates and a pack of heat transfer plates arranged between said plates. The two plates of the frame comprise a pressure plate, which is movable during assembly, and a frame plate, which is fixed during assembly, said plates being pulled together by means of bolts, thereby clamping the heat transfer plates. For the sake of simplicity, the pressure plate and the frame plate will both be referred to as end plates below. The number of heat transfer plates as well as their size is determined by the field of use of the plate heat exchanger. One of the end plates, or both, is provided with through port openings to allow inflow and outflow of a number of (usually two) heat exchanging fluids. The heat transfer plates are, in turn, provided with a number of through ports, which form a first inlet channel and a first outlet channel for a first fluid through the plate pack and a second inlet channel and a second outlet channel for a second fluid through the plate pack. These channels extending through the plate pack communicate with the through port openings of the end plates.
The heat exchanging fluids flow separately through the plate heat exchanger in different plate interspaces formed between the heat transfer plates. In most cases, every second such plate interspace communicates with the first inlet and outlet channel, each plate interspace being adapted to define a flow area and to conduct the first heat exchanging fluid between said inlet and outlet channels. Correspondingly, the other plate interspaces communicate with the second inlet and outlet channel for a flow of the second heat exchanging fluid. Fluid-tight sealing means such as a gasket or weld are provided round the through ports of the heat transfer plates. The sealing means are arranged round some of the ports alternatingly in every second plate interspace and, in the other plate interspaces, round the other ports so as to form the two separate channels for the first heat exchanging fluid and the second heat exchanging fluid, respectively.
Since the purpose of a plate heat exchanger is to achieve a heat exchange between two fluids, both the end plates and the heat transfer plates are subjected to a significant influence of temperature. This influence causes problems, as will be described below.
The heat transfer plates are usually relatively thin and are in direct contact with the heat exchanging fluids. The temperature of the fluids will thus directly affect the temperature of the heat transfer plates, the length of which will change to a certain extent depending on the coefficient of linear expansion of the plate material.
The end plates, which are located on either side of the pack of heat transfer plates, are considerably thicker than the heat transfer plates. Moreover, the end plates do not enter into direct contact with the heat exchanging fluids as do the heat transfer plates. However, the temperature of the end plates will be affected on one side of the end plate by the environment surrounding the plate heat exchanger and, on the other side, by the temperature of the respective outermost heat transfer plates in the plate pack.
The changes in length will vary due to the difference in the influence of temperature between the end plates and the heat transfer plates in combination with different plate thickness. In addition, the different length changes of the end plates and heat transfer plates may be reinforced by the fact that the plates are often made of different materials having, thus, different coefficients of linear expansion. The different length changes of the plates result in high tensions in the attachment of the connections between the end plates and the heat transfer plates, which leads to an increased risk of fatigue damage.
However, the primary cause of fatigue damage is the difference in thermal inertia between the end plates and the heat transfer plates. A rapid change in the temperature of the fluids will cause the temperature of the heat transfer plate to change immediately whereas the temperature of the end plate will change very slowly. In many processes, temperature variations occur during operation, for example in batch processes. In batch processes, a certain amount of components, such as fluids, powder or pellets, is processed for a certain period of time, following which the process is interrupted to allow emptying, cleaning and charging of a new batch. Thus, a batch process involves many starts and stops in connection with which the temperature changes from a maximum value to a minimum value.
Fatigue damage in the attachment of the connections between the end plates and the heat transfer plates is due to a number of factors and may lead to fracture in the material and thus to a shortened service life of the plate heat exchanger.
To compensate for the different changes in length of the end plates and the heat transfer plates, it is proposed in U.S. Pat. No. 6,119,766 to arrange one or more bellows on the plate heat exchanger. The bellows are connected to an end plate provided with port openings and to the associated outer heat transfer plate and are adapted to absorb any movements between the frame and the pack of heat transfer plates.
However, there are several fields of application in which a plate heat exchanger provided with bellows as described above cannot be used. For example, the bellows design does not allow high pressures to be used in the plate heat exchanger. To withstand high pressures, in the range of 100–150 bars (10–15 Mpa), the thickness of the material of which the bellows are made must be great, which means that the bellows will be rigid. However, such a rigid design means that the bellows lose their flexibility and thus their capacity to absorb movement between the end plates and the heat transfer plates in a satisfactory manner.
Furthermore, a plate heat exchanger provided with a bellows cannot be used, for example, in certain types of chemical applications where specific materials that are resistant to chemical attack must be used. In most cases, no such bellows are available since different kinds of ceramic materials are often used to obtain the chemical durability. Ceramic materials are usually brittle and cannot be used in bellows of the kind described in U.S. Pat. No. 6,119,766.
JP 2000 329493 discloses a plate heat exchanger comprising an end plate provided with slits. The slits are adapted to absorb small deformations when a thermal stress is applied to both the inlet and the outlet holes on the end plate. The end plate, which for reasons related to manufacture, no doubt, is made in one piece, is provided with two parallel slits. The slits extend from the opposite longitudinal edges across the main portion of the width of the end plate adjacent to the outlet/inlet holes.
The design of the slits in JP 2000 329493 allows only small deformations of about 1/100 mm to be absorbed. Larger deformations will cause cracks to appear at the extreme ends of the slits and the end plate will thus be damaged.
Accordingly, the plate heat exchanger design described in JP 2000 329493 cannot be used in large plate heat exchangers, in which the thermal deformations may be several millimetres.
Various designs of plate heat exchangers and of parts included therein have been known for a long time. One example is EP 033,201, which discloses a frame for a plate heat exchanger. In conventional manner, the frame is made up of two end plates, which however in turn are divided into a number of units. The purpose of this division of the end plates into units is on the one hand to allow simpler and more rational manufacturing and, on the other hand, to facilitate the handling and assembly of the end plates and the plate heat exchanger. In order to serve as conventional end plates, the different units are assembled into rigid plates in connection with the assembly of the plate heat exchanger.
Thus, there is currently no plate heat exchanger concept in which the plate pack is clamped between two end plates and which can be used in a satisfactory manner under the conditions described above, for example at high pressures and in a chemically aggressive environment, and which can absorb considerable thermal deformations.